1. Field of Invention
Exemplary aspects of the present invention relate to an image processing method or the like for use in a digital printer, such as an ink jet printer and a laser printer, and an image forming apparatus, such as a digital copying machine, and more particularly to an image processing method, an image processing apparatus, an image processing program, a printer, a print instructing terminal, and an image processing system to achieve binarization of a multi-gradation image having half-tone grayscale at high speeds with the use of the error diffusion method.
2. Description of Related Art
In the related art the threshold method, the dither method, and the error diffusion method have been used to subject multi-gradation image data having half-tone grayscale, such as a full color image, to binarization.
The error diffusion method enables a pseudo half-tone grayscale image to be expressed by sequentially diffusing (distributing) an error value, which is generated when each pixel of interest undergoes binarization (for example, whether the pixel is white or black, whether the drawing takes place or not, whether toner or ink is to be ejected or not, etc.) by the threshold method, to pixels on the upper stream, specifically, to unprocessed pixels, according to a predetermined diffusion table. The diffusion processing, however, requires large information processing ability in, for example, CPU power and memories. Thus, speeding up of the processing has been a challenge.
As a method to speed up binarization with the use of the error diffusion method as described above, there is a related art method, by which a multi-gradation image to be processed is divided into several regions in advance. Then, the respective divided regions are processed in parallel. When the processing ends, all the divided regions are synthesized (joined) to a single image.
According to the method for enabling parallel processing by dividing one image as described above, however, the error diffusion processing is not applied normally to a dividing line portion. Thus, when the respective divided images are joined, discontinuous points are generated in the joined portion, and become noticeable as stripes. This poses a problem that the image quality is impaired markedly.
For example, JP-A-6-301364 aims to suppress the occurrence of stripes in the joined portion of an image by devising the diffusion method in such a manner that an input image to be processed is divided vertically in the main scanning line direction. When the error diffusion processing is applied to the respective regions in parallel, errors of pixels bordering upon the boundary line are diffused to nearby pixels in an adjacent image region with respect to these bordering pixels.
Also, JP-A-11-17945 attempts to prevent deterioration of the image quality in the joined portion of an image by further lagging the binarization in an adjacent region at least one line behind to store errors in the boundary region, so that the stored errors are diffused to pixels in the adjacent region which has not been processed yet.
Further, JP-A-9-284543 aims to make stripes in the joined portion less noticeable by processing the image regions that have undergone binarization independently to overlap, and by applying special processing to the joined portion.